The present invention relates to precision rocker mills and, more particularly, to precision rocker mills of the type which produce tubes from hollow metal workpieces.
Rocker mills of the type known in the art are disclosed, for example, in U.S. Pat. Nos. 4,562,713 to Kondoh and 4,930,328 to Duerring et al. Such mills typically include a movable rollstand which is longitudinally reciprocated along a hollow workpiece. The rollstand includes a pair of grooved roll dies which define a nip through which the workpiece is passed so as to be radially compressed. A mandrel is disposed within the workpiece to radially support the inside of the workpiece. The workpiece is progressively advanced and rotated at selected times.
The grooves of the roll dies are of progressively narrowing width in the circumferential direction. The grooves typically extend around the entire circumference of the roll die. A relief pocket is formed in the grooves and defines a transition from a smaller portion to a larger portion. The relief pocket is provided to facilitate reversing the direction of rotation of the roll dies and advancing and rotating the workpiece. Thus, by rotating the roll dies so that points on the periphery of the dies are displaced through the angle defined by the groove as the rollstand moves longitudinally along the workpiece, the workpiece is subjected to a progressively increasing radial compression in the nip between the dies and the diameter of the workpiece is progressively reduced.
During a workpiece reduction stroke of each cycle of the rolling operation, the roll dies are rotated such that the narrowest portion of the grooves define the nip. At the final portion of the reduction stroke, the workpiece is usually positioned between the dies in a nip defined by the relief pocket. After the roll dies reach the end of the reduction stroke, the roll dies are rotated in an opposite direction in a smoothing stroke of the cycle of the rolling operation. During the smoothing stroke, the nip defined by the grooves becomes larger and the surfaces of the grooves are moved away from the reduced diameter workpiece. At the end of the smoothing stroke, the workpiece is again usually positioned between the dies in a nip defined by the relief portion and the workpiece is advanced a predetermined distance and is rotated axially through a predetermined angle.
During the reduction stroke, the workpiece tends to be deformed by the roll dies into the shape of an ellipse having a major axis along the nip between the roll dies. The workpiece is rotated prior to the smoothing stroke so that the cumulative effect of numerous deformations of the workpiece is to form a substantially circular workpiece. If the workpiece is not rotated, the effect of the rolling operation is typically to form a finned tubular product. It is preferred that the angle of rotation of the workpiece not be divisible into 360.degree. without a remainder, and the angle of rotation is generally between 35.degree. and 70.degree..
Rotation of the workpiece is typically accomplished by one or more workpiece turners. Turners generally include a clamp which grips the exterior of the workpiece at some point before or after the path of travel of the roll dies. Types of turners typically used in conventional rocker mills include feed carriage turners and inlet turners for turning the workpiece as it is fed between the rollers. An exit turner is generally provided for turning the workpiece after it has been reduced in diameter and its trailing end has come out of contact with the inlet turner. A mandrel turner is provided for turning the mandrel that is disposed inside the workpiece when the workpiece is turned.
When rocker mills are operating on tubing such as thin-walled tubing and small diameter tubing, the use of turners often causes damage to the tubing, such as where the tubing is gripped too tightly and the tubing walls are bent far punctured or the tube is twisted. Conventional rocker mills using one or more workpiece turners have various disadvantages. For example, complex equipment is necessary to synchronize multiple turners. Further, when it is desired to change the size of the workpiece and the product, it is necessary to change the jaw size on all of the turners. Further still, the need to turn the workpiece at the end of the reduction or smoothing stroke slows down the operation of the rocker mill, as it generally takes substantially longer to turn the workpiece than it does to incrementally advance it. Other disadvantages in the use of a rocker mill in which a workpiece is turned arise during the use of the rocker mill in association with other workpiece handling equipment, such as product coilers. For example, if it is desired to coil a product, the coiler must be rotated as the workpiece is rotated so that the product does not become twisted.
It is desirable, therefore, to provide a rocker mill in which the need for turners is obviated while still producing a substantially circular reduced product.
The present invention, generally speaking, obviates the need for workpiece and mandrel turners in an apparatus and method for reducing the diameter of a cylindrical workpiece.
In accordance with one aspect of the present invention, a rocker mill for reducing the diameter of a cylindrical workpiece is provided. The rocker mill includes a frame and an assembly including means for supporting a pair of roll dies rotatably about their axes. The roll dies have tapered circumferential grooves defining a nip for receiving a workpiece. Means for supporting the assembly relative to the frame are provided. The assembly supporting means permit the assembly to rotate about the longitudinal axis of the workpiece and permit the assembly to move longitudinally relative to the frame. Means are provided for longitudinally reciprocating the assembly including the roll dies relative to the frame through a predetermined distance through a reduction stroke and a smoothing stroke. Intermittent rotating means are provided for rotating the roll dies around the longitudinal axis of the workpiece such that rotation occurs through a predetermined angle.
In accordance with another aspect of the present invention, a method for reducing the diameter of a cylindrical workpiece is described. In the method, a workpiece is fed between a pair of roll dies having tapered circumferential grooves defining a nip for receiving the workpiece. The roll dies are longitudinally reciprocated relative to the workpiece through a predetermined longitudinal distance through a reduction stroke and a smoothing stroke such that the roll dies rotate about roll die axes. The roll dies are intermittently rotated around the longitudinal axis of the workpiece such that rotation occurs through a predetermined angle.
In accordance with yet another aspect of the present invention, a rotatable rollstand assembly for a rocker mill is described. The rotatable rollstand assembly includes a die assembly including axle means for rotatably supporting a pair of roll dies having tapered circumferential grooves defining a nip for receiving a workpiece and a pair of gears. Means, including a rack assembly, are provided for supporting the die assembly relative to a pair of racks for engaging with the pair of gears such that longitudinal movement of the die assembly relative to the racks causes the roll dies to rotate in opposite directions. At least portions of the supporting means are adapted to be rotatably mounted such that rotation of the supporting means causes rotation of the roll dies about a longitudinal axis passing through the nip.